One of the major factors determining the strength and extent of ENSO events, is the instability state of the equatorial Pacific coupled ocean-atmosphere system and its seasonal variations. This study analyzes
the coupled instability in a hybrid coupled model of the Indo-Pacific region, using the adjoint method for sensitivity studies.
It is found that the seasonal changes in the ocean-atmosphere instability strength in the model used here are related to the outcropping of the thermocline in the east equatorial Pacific. From July to December, when the thermocline outcrops over a wide area in the East Pacific, there is a strong surfacethermocline connection and anomalies that arrive as Kelvin waves from the west along the thermocline can reach the surface and affect the SST and thus the coupled system. Conversely, from February to June, when the thermocline outcropping is minimal, the surface decouples from the thermocline and
temperature anomalies in the thermocline depth range do not affect the surface and dissipate within the thermocline. The role of vertical mixing rather than upwelling in linking vertical thermocline movements
to SST changes is emphasized.
It is therefore suggested that the seasonal ocean-atmosphere instability strength in the equatorial Pacific is strongly influenced by the thermocline outcropping and its seasonal modulation, a physical mechanism that is often neglected in intermediate coupled models and that can be represented properly only in models that employ the full dynamics of the mixed-layer.